1. Field of the Invention
The present invention relates to a piezoelectric substance used in a liquid discharge device, a piezoelectric substance element, a liquid discharge head and a method for producing them. More specifically, the present invention relates to the piezoelectric substance element of a large area and high density, the liquid discharge head and the method for producing them.
2. Description of the Related Art
Higher resolution, and higher-speed printing, and further a longer length of a head have been demanded to an inkjet printer. For the purpose, a multi-nozzle head structure with a refined head has been demanded. It is necessary to miniaturize a piezoelectric substance element for discharging a liquid in order to refine the liquid discharge head, and further it is required for the piezoelectric substance to have such a high piezoelectric constant as not to decrease driving ability even when being refined, in order to miniaturize the piezoelectric substance element. For this reason, when the piezoelectric substance employs a piezoelectric substance film, the piezoelectric substance film is required to have superior crystallinity, in other words, to have the crystallinity controlled so as to make crystals highly oriented. In order to give the piezoelectric substance film the highly oriented crystallinity, it is preferable that an underlayer of a prepared piezoelectric substance film has higher crystallinity, and that the piezoelectric substance film and the underlayer have an adequate compatibility of lattice with each other. Besides, the piezoelectric substance film is required to have adequate adhesiveness to the underlayer so as not to cause peeling between the film and the underlayer, because the peeling tends to occur due to the stress applied in an interface, which has increased as the piezoelectric substance film becomes thin. A PZT piezoelectric material has been heretofore used for the piezoelectric substance film in the piezoelectric substance element, which is obtained by the steps of molding a paste of powders of PbO, ZrO2 and TiO2 into a sheet shape to form a green sheet, and sintering it, as is described in Japanese Patent Application Laid-Open No. S62-213399.
In addition, as for an piezoelectric material having higher piezoelectric characteristics than the above-described PZT piezoelectric material, there is a relaxor material such as Pb (M1/3N2/3) O3 (M=Mg, Zn, Co, Ni, Mn, N=Nb, Ta), Pb (M1/2N1/2) O3 (M=Sc, Fe, In, Yb, Ho, Lu, N=Nb, Ta), Pb (M1/2N1/2) O3 (M=Mg, Cd, Mn, Co, N=W Re) and Pb (M2/3N1/3) O3 (M=Mn, Fe, N=W, Re), as is disclosed in US2005/0122005A1.
However, a PZT oxide film according to a method disclosed in the above described Japanese Patent Application Laid-Open No. S62-213399 is hardly formed into a thickness of, for instance, 10 μm or smaller. In addition, the PZT oxide film has a problem that the piezoelectric substance film shrinks to about 70% when heated, because a green sheet is sintered at 1,000° C. or higher. Accordingly, the piezoelectric substance film is hardly aligned with a structure like an ink chamber in a dimensional accuracy of an order of several microns, so that a satisfactorily small piezoelectric substance element has not been realized yet.
Furthermore, a ceramic piezoelectric substance film formed by sintering a green sheet could not provide adequate piezoelectric characteristics, because as the film becomes thin, an effect of a crystal grain boundary becomes not negligible. As a result, even if the piezoelectric substance film with a thickness of 10 μm or less were prepared by sintering the green sheet, the piezoelectric substance film had a problem of not showing sufficient piezoelectric characteristics for discharging a recording liquid.
A thin film of 10 μm or smaller can be obtained by a sputtering method, a CVD method, a MBE method and a sol-gel process. However, the piezoelectric substance film produced by these methods has a problem of showing degraded adhesiveness of the film to a lower layer of a lower electrode, because the film acquires high density and consequently causes high in-plane stress in itself. A piezoelectric substance element of an inkjet head needs to have high adhesiveness between the piezoelectric substance film and the lower electrode of the lower layer, in order to bear stress generated by repetitive drive.
On the other hand, a relaxor material as is disclosed in US2005/0122005A1 has high piezoelectric characteristics, but the piezoelectric characteristics are highly temperature dependent, and the relaxor material also has a low Curie temperature. Accordingly, a piezoelectric element using the relaxor material needs to keep an operation environmental temperature constant, which does not meet reality and the piezoelectric element having low temperature dependency has not yet been in actual use.